1. Field of the Invention
The present invention relates to the control of airflow between adjacent sections of a paint booth.
2. Description of the Prior Art
Durable goods such as vehicles and appliances require protective coatings. Generally, protective coatings are added to such objects from inside the protective environment of multi-section enclosures commonly referred to as coating or paint booths. Typically, coatings of several types and pigments are applied to objects as they are conveyed through a paint booth. Usually, only one type of coating is applied per section of paint booth. For example, a first section may be used to apply a primer coat, another section may be used to apply a pigment and a final section could be used to apply a clear coat.
No matter what type of coating is being applied, a clean environment inside each paint booth section free of detrimental substances such as dirt, dust and organic solvents must be maintained. The paint booth enclosure alone limits to some degree the amount of coating contaminants present inside each paint booth section. Never the less, a significant amount of contaminants still exist inside each paint booth section. These contaminants must be prevented from coming into contact with freshly applied coatings.
One way to prevent contaminants from coming in contact with fresh uncured coatings is to force clean air to flow vertically from vents in the top of the paint booth to returns in the bottom. Downward airflow helps prevent contaminants from becoming suspended inside the paint booth.
Unfortunately, the downward airflow used to solve the problem of suspended contaminants creates another problem by generating unequal static pressures between adjacent sections. The higher static pressure of one paint booth section relative to another forces airborne paint particles to migrate to the lower pressure section. Walls placed between adjacent sections only moderately reduce the number of paint particles transported because the walls must have openings through which objects can pass.
Transported paint particles become coating contaminants when they drift into other sections. For example, white-pigment paint particles migrating from one paint booth section into another containing an object with an uncured black-pigment finish coat would be disastrous.
Prior art airflow control systems have been designed to minimize paint particle migration between adjacent sections of a paint booth. One such system uses ultrasonic anemometer sensors to measure airflow between adjacent paint booth sections. The airflow measurements are sent to an air handling system that increases or decreases the airflow in each paint booth section. Unfortunately, ultrasonic anemometers cannot accurately measure airflow rates as low as those that transport paint particles between sections.
Another system uses pressure sensors to measure pressure differences between adjacent paint booth sections. An air handling system responds to the pressure sensors by adjusting the airflow of each section. While this approach is sound in theory, it is unsatisfactory in a practical sense because factory calibrations of low-pressure sensors are very short-lived. In other words, the output of low pressure sensors significantly drift off calibration unacceptably soon, especially when set to measure differential pressures as low as those responsible for the transport of the undesired paint particles.
As described above, the purpose of paint booths are to prevent the contamination of uncured coatings by providing an environment free from particles nd substances that would otherwise mix with the coatings applied to an object, harming the object""s finish. While the enclosure of a paint booth along with the introduction of clean air into its sections go towards providing an environment suitable for applying coatings to durable goods such as vehicles and appliances the problem of paint migration between adjacent sections remains. A solution to this problem will provide a major benefit in that costly rework and refinishing of coated objects now common will be eliminated.
It has been found that the migration of undesirable paint particles from one paint booth section to another can be significantly reduced by maintaining substantially equal static pressures between paint booth sections. Pressure sensors are used to measure the static pressure inside paint booth sections. An airflow controller uses static pressure measurements from the paint booth sections to determine the level of airflow adjustment needed to equalize the pressure in adjacent sections. Frequent calibration of all pressure sensors during the continuous operation of a paint booth is necessary because pressure sensors rapidly drift away from calibration when used to make measurements with the accuracy needed to determine airflow adjustments.
The present invention is a system for calibrating pressure sensors used to measure the static pressure inside adjacent paint booth sections. The system includes a pressure sensor, an amplifier and an amplifier controller for each paint booth section. A processor connects a low-pressure reference and a high-pressure reference to each of the pressure sensors in a sequential manner. The amplifier controller adjusts the amplifier output to zero based on the low-pressure reference. Likewise, the amplifier controller adjusts the amplifier to a preset value based on the high-pressure reference. When referring to xe2x80x9czeroxe2x80x9d as a procedure it is to be understood that it is unnecessary for any output to physically go to zero volts or zero amperes, etc. Instead the term xe2x80x9czeroxe2x80x9d in the case of the present invention includes multiple points of calibration. For example, to xe2x80x9czeroxe2x80x9d an output the user may select a voltage set point of perhapsxe2x80x94100 mV.
The present invention further provides an airflow controller that uses the static pressure measurements from inside paint booth sections to determine the level of airflow needed to reach static pressure equilibrium between adjacent paint booth sections. Each amplifier is in communication with an airflow controller that may be as simple as a motor controller in command of a blower. The blower in turn generates airflow into the booth section in which pressure is measured by a corresponding pressure sensor. An increase in airflow entering or a decrease in the airflow exiting a booth section will increase the pressure inside the section. Likewise, a decrease in the airflow entering or an increase in the airflow exiting a booth section will result in a decrease in pressure inside the section. Consequently, the pressure inside the booth can be maintained at a level that is equal to that of neighboring booth sections. Equalized pressured between adjacent sections results in minimized airflow between adjacent sections.
Also, a method is disclosed in which a micro-controller controls the sequence of automatic zero and automatic span procedures that recalibrate all pressure sensors on an automatically adjustable time schedule. The pressure sensors are recalibrated frequently, in both zero offset and span; as a result all sensors output equivalent signals for an equivalent pressure. This level of accuracy and equivalence of pressure measurement an air flow controller can be set to maintain the pressure of each paint booth section such that pressure differences between sections is reduced to a level that drastically reduces the transport of paint particles between sections.